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UCRL-JRNL-203401

Increase of Carbon Cycle Feedback with Climate Sensitivity: Results from a coupled Climate and Carbon Cycle Model

B. Govindasamy, S. Thompson, A. Mirin, M. Wickett, K. Caldeira, C. Delire April 6, 2004

Tellus B

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UCRL-JRNL-203401

Increase of Carbon Cycle Feedback with Climate Sensitivity: Results from a Coupled Climate and Carbon Cycle Model
B. Govindasamy, S. Thompson, A. Mirin, M. Wickett, K. Caldeira , and C. Delire* Climate and Carbon Cycle Modeling Group Lawrence Livermore National Laboratory Livermore, CA 94550 *Center for Sustainability and the GlobalEnvironment Gaylord Nelson Institute for Environmental Studies, University of Wisconsin-Madison Abstract Coupled climate and carbon cycle modeling studies have shown that the feedback between global warming and the carbon cycle, in particular the terrestrial carbon cycle, could accelerate climate change and result in larger warming. In this paper, we investigate the sensitivity of this feedback foryear-2100 global warming in the range of 0 K to 8 K. Differing climate sensitivities to increased CO2 content are imposed on the carbon cycle models for the same emissions. Emissions from the SRES A2 scenario are used. We use a fully-coupled climate and carbon cycle model, the INtegrated Climate and CArbon model (INCCA) the NCAR/DOE Parallel Coupled Model coupled to the IBIS terrestrial biosphere modeland a modified-OCMIP ocean biogeochemistry model. In our model, for scenarios with year-2100 global warming increasing from 0 to 8 K, land uptake decreases from 47% to 29% of total CO2 emissions. Due to competing effects, ocean uptake (16%) shows almost no change at all. Atmospheric CO2 concentration increases were 48% higher in the run with 8 K global climate warming than in the case with nowarming. Our results indicate that carbon cycle amplification of climate warming will be greater if there is higher climate sensitivity to increased atmospheric CO2 content;

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UCRL-JRNL-203401

the carbon cycle feedback factor increases from 1.13 to 1.48 when global warming increases from 3.2 to 8 K.

Introduction The physical climate system and the global carbon cycle are tightly coupled, aschanges in climate affect exchange of atmospheric CO2 with the land surface and ocean, and changes in CO2 fluxes affect Earth’s radiative forcing and the physical climate system. During the 1980s, oceanic and terrestrial uptake of carbon amounted to a quarter to a third of anthropogenic CO2 emissions with strong interannual variability (Braswell et al., 1997; Prentice et al., 2000; 2001). Abetter understanding of carbon balance dynamics is required for interpreting variations in atmosphere-biosphere exchange (Fung et al., 1997) and for evaluating policies to mitigate anthropogenic CO2 emissions (United Nations Framework Convention on Climate Change 1997; IGBP Terrestrial Carbon Working Group 1998). Anthropogenic emissions of fossil fuels and land use change are expected to lead to...